Conditioned insomnia: An insomnia that is produced by the development of conditioned arousal during an earlier experience of sleeplessness. A component of psychophysiological insomnia.

Delta sleep: Also known as deep sleep, classified as N3 stage of NREM sleep.

Delayed Sleep Phase Syndrome (DSPA): A circadian rhythm sleep disorder where in the sleep phase occurs well beyond the conventional bed time with delayed wake up time. Seen in teenage groups and young adults (Night owl).

Diurnal: Pertaining to daytime.

Epworth Sleepiness Scale (Epworth Sleepiness Score): A subjective assessment of severity of daytime sleepiness. A very commonly used measure of excessive daytime sleepiness.

Final awakening: The amount of wakefulness that occurs after the final wake-up time until the arise time (lights on).

Final wake-up: The clock time at which an individual awakens for the last time before the arise time.

First night effect: The effect of the environment and Polysomnographic recording apparatus on the quality of the subjectâ€™s sleep the first night or recording.

Free running: A Chronobiological term that refers to the natural endogenous period of a rhythm when zeitgebers are removed.

Hypercapnia: Elevated carbon dioxide level in blood.

Hypersomnia: Excessive sleepiness

Hypnogram: A graphic representation of various stages of NREM and REM sleep.

Hypnogogic hallucinations: Hallucinations occurring at the transition from wakefulness to sleep and or at sleep onset.

Hypnopompic hallucinations: Hallucinations occurring at the transition from sleep to wakefulness or at the offset of sleep.